1. Field of the Invention
The present invention broadly relates to a method of forming images and to a belt transport device including a drive roller and a belt wound around the drive roller, and a belt fixing device using a belt transport device, and further relates to a belt fixing device to fuse an unfixed toner image on a recording member bearing the unfixed toner image via heat and pressure in an image forming apparatus of an electrophotographic type.
2. Description of the Related Art
Belt transport devices are used in various technological fields today. For example, image forming apparatuses of the electrophotographic type such as copiers, printers and the like are provided with fixing devices to fuse an unfixed toner image on a recording member bearing the unfixed toner images under heat and pressure, and such fixing devices may use a belt transport device as the fixing device.
Belt fixing devices are provided with a fixing roller/drive roller heated by a heater, an endless belt wound around the rollers, and a back-up member (typically a back-up roller) pressing against the drive roller through the endless belt. Fixing devices of this type are advantageous compared to fixing devices having a construction providing a back-up roller disposed opposite a heating roller heated by a heater; the related advantages are described below.
Heating an unfixed toner image formed on the surface of a recording member starts when it is inserted between the fixing roller and the back-up roller without any preheating, such that the toner is inadequately heated on the recording member side, although a predetermined temperature on the surface side of the toner image is attained; thereby, making it difficult to achieve excellent toner image fusion. This tendency is particularly pronounced when toner images comprising cyan, magenta, yellow, and black are overlaid one over another on the surface of a recording member in color image forming apparatuses. When a belt fixing device is used, however, an unfixed toner image can be preheated before insertion at the fixing position by means of the heated belt extending upstream from the fixing position in the direction of the transport of the recording member, so as to achieve the desired toner fusion under heat and pressure.
Consideration has been given to elevating the fixing temperature without using a belt fixing device. However, when the fixing temperature is raised, molten toner readily adheres to the fixing roller causing offset. A sufficient application of a separation agent, such as silicone oil, or the like, must be applied to the fixing roller so as to prevent this offset. This leads to further disadvantages insofar as excessive application of the separation agent will cause the separation agent to be transferred to the recording member, thereby reducing the quality of the recording member. Therefore, simply elevating the fixing temperature is not a sufficient solution.
Through extensive investigation, the present inventors found several problems with conventional belt fixing devices.
In belt fixing devices used in image forming apparatuses of an electrophotographic type, a recording member must be smoothly transported during the fixing of a toner image so as to avoid disturbing and dislocating the unfixed toner image carried on the surface of the recording member, and therefore, the recording member must be transported at a predetermined speed. According to the research conducted by the present inventors, when an unfixed toner image is fused onto the surface of a relatively thick recording member such as, an envelope, the fixing pressure must be reduced to avoid causing wrinkles in the recording member. However, reducing the pressure adversely affects the fixing characteristics of the toner image. It was discovered that forming at least a surface layer of an elastic material on the fixing roller/drive roller and increasing the fixing nip width between the fixing roller/drive roller and the back-up member, produces excellent toner image fusion while reducing the fixing pressure to a particular degree.
However, if the surface layer of a fixing roller/drive roller is formed of an elastic material in a belt fixing device, the radius of the drive roller readily changes due to the soft irregularities of the surface layer and tearing off of protuberances of the surface layer, so as to make it difficult to maintain the belt transport speed at a predetermined speed. This ultimately causes the recording member transport speed to fluctuate so as to easily reduce the quality of the fixed image.
Although these disadvantages have been described in terms of belt fixing devices used in image forming apparatuses of an electrophotographic type, such disadvantages are not limited to belt fixing devices. Rather, such belt fixing devices running a belt at a predetermined speed with good precision and transporting a transport object with as little damage as possible to the object, are applicable to belt transport devices in general which grip and transport a transport object between an endless belt and back-up member and are provided with a drive roller, driven roller, endless belt wound around the drive roller and driven roller, and back-up member pressed against the drive roller through the endless belt.